What storage size is right for your household?

This is how you determine a meaningful storage capacity based on your electricity consumption, solar system, load profiles, and personal goals.

Not as large as possible, but appropriately sized

An electricity storage system is considered appropriately sized when it can be regularly charged with available solar power and discharged again by your actual consumption. A very large storage system does not automatically offer greater benefits. If a significant portion of its capacity remains unused on many days, acquisition costs increase without a proportional increase in self-consumption. Conversely, a storage system that is too small quickly reaches its capacity limit and cannot absorb a portion of the midday solar surplus.

Therefore, for selection, you should consider three quantities separately: Energy in kilowatt-hours describes how long the storage system can supply your household. Power in watts or kilowatts describes how many consumers can be supplied simultaneously or how quickly it can be charged and discharged. Usable capacity refers to the actually available portion of the stated storage amount. Nominal capacity and usable capacity can differ.

DRBO Greenenergy recommends not basing the decision on a single rule of thumb. Your daily consumption, consumption times, the size and orientation of the solar system, possible shading, and your goal are decisive: Do you primarily want to use solar power in the evening, cover peak loads, or prepare for a later expansion?

Determine the actual energy demand

Start with your annual consumption from your last electricity bill. Divide this value by 365, and you get an average daily consumption. A household with 3,650 kWh per year theoretically consumes about 10 kWh per day. However, this average value is only a starting point, as winter, summer, weekdays, and weekends differ significantly.

More important than the mere daily value is consumption outside of solar hours. Check how much electricity is typically needed in the late afternoon, evening, night, and early morning. Precisely during this period, the previously stored solar power should cover as large a portion of your needs as possible. Consumers such as heat pumps, electric cars, hot water heaters, air conditioners, or continuously running IT can significantly change the load profile.

• Base load: Refrigerator, router, stand-by devices, and home technology run for many hours with comparatively low power.
• Evening load: Cooking, lighting, entertainment, and household appliances often create the greatest demand after sunset.
• High individual consumers: Kettles, ovens, or chargers require a lot of power in the short term, but not necessarily a lot of energy.
• Shiftable loads: Washing machines, dishwashers, or vehicle charging can often be consciously shifted to solar hours, thereby reducing the necessary storage requirement.

If an energy meter or a smart meter provides quarter-hourly or hourly values, use this data. They show much more precisely how much energy is needed between sunset and the next solar yield.

Realistically assess solar yield and usable surplus

A storage system can only absorb energy that is actually generated and not consumed at the same time. The module output alone, therefore, does not indicate how much energy is available daily for charging. Orientation, inclination, location, season, temperature, and shading influence the yield. Inverter limitations and system losses must also be considered.

For an initial assessment, compare typical daily yields of your system with consumption during sunny hours. The difference is the possible surplus. An example: If the system generates 6 kWh on a good day and the household immediately consumes 2 kWh of that, theoretically up to 4 kWh are available for storage. In practice, conversion and storage losses reduce this value. On cloudy days or in winter, the surplus can be significantly smaller.

Size the storage primarily for frequently occurring days in spring, summer, and autumn. Sizing solely for maximum summer yield often leads to unused capacity; sizing solely for winter is also not very sensible if there is hardly any solar surplus.

Evaluate capacity, power, and expandability together

The appropriate capacity answers the question of how many kilowatt-hours you want to shift in time. The output power, on the other hand, answers which consumers the storage can support simultaneously. A storage system with a large capacity but low output power can cover a base load for a long time, but cannot automatically supply several high-power devices simultaneously. Conversely, high power with small capacity can only be available for a short time.

Also, check the maximum permissible charging power. With a high available PV power, a low charging power can lead to short-term yield peaks not being fully stored. For small systems, very high charging power is rarely the decisive advantage. Pay attention to the values of the complete system and not just individual components.

• Usable capacity: Compare the actually available energy content.
• Continuous power: Check which load can be supplied continuously.
• Peak power: Evaluate short-term loads only according to the specific manufacturer's specifications.
• Modularity: Clarify whether and under what conditions expansion modules may be added.
• System limits: Observe maximum number of modules, total voltage, current, firmware versions, and approved combinations.

A modular solution can be useful if your future consumption is still uncertain. You then start with a conservative capacity and expand based on real consumption data. Before purchasing, check whether a later expansion is technically supported and whether identical generations, capacities, or software versions are required.

A reliable decision in five steps

For an initial selection, you can proceed systematically. First, you record annual consumption and daily profile. Second, you determine typical consumption outside solar hours. Third, you estimate the regularly available solar surplus. Fourth, you check which continuous power your important consumers require. Fifth, you compare these values with the usable capacity, charging and discharging power, and expandability of the system.

A simplified example calculation: If your household typically needs 3 kWh in the afternoon, evening, and night, and about 2.5 to 4 kWh of solar surplus are available on many suitable days, a usable storage capacity in this range is closer to the real demand than a significantly larger system. This is not a universal product recommendation, but a method for narrowing down options. Peak loads, winter operation, and individual goals must also be considered.

Document your assumptions and check after a few weeks whether they match the measurement data. If you are between two sizes, the larger variant is not automatically correct. An expandable solution, altered load management, or additional solar modules can be more economically and technically sensible. DRBO Greenenergy can check your consumption data and the planned system configuration; the final selection should always be based on the technical documents of the components involved.

The most important factors for your decision

The most important thing: Base the storage size on the regularly available solar surplus and your consumption outside of solar hours. Evaluate capacity, continuous power, charging power, and expandability together. A realistically utilized, technically suitable solution is usually more sensible than the largest possible capacity.